Biosensor slashes time to detect Listeria bacteria

Portland, Ore. - A fiber-optic biosensor that reduces testing time for a deadly form of the Listeria bacteria from one week to less than 24 hours has been created by two Purdue University microbiologists and a control engineer.

Listeriosis kills one in five that it infects and accounts for the highest rate of hospitalization and mortality among foodborne illnesses. Usually, when the (monocytogenes) Listeria bacteria is detected in a food sample, it has already been on store shelves for up to a week, because the detecting sensors are so crude that the food has to be cultured in petri dishes for that long before enough bacteria multiply to trip the alarm.

"We still need to culture the food sample with our new biosensor, but for a much shorter time-less than a day," said Arun Bhunia, associate professor of food microbiology. Bhunia developed the sensor with Tao Geng, research associate in the Department of Food Science. Mark Morgan, an engineering professor in Purdue's Food Science Sensors and Controls Laboratory, also participated in the research.

"Instead of having to recall food that is already on the shelves, we want test results before it gets delivered to stores," said Bhunia.

The researchers are aiming eventually for real-time operation of the biosensor.

A researcher gets the sensor to work by coating the fiber-optic cable with an antibody that can only attach to the pathogen in question. Any Listeria bacteria present will stick to the fiber-optic cable when it is poked into the food product. The cable is then removed and dipped in a fluorescent-marker solution that causes the optical fiber to glow when any Listeria is present.

After the sensor is used, it can still take up to a day to get a report on the results, but the Purdue researchers are looking to "refine the engineering to make our sensor more sensitive," said Bhunia.

Inside scoopIn the world of microbiology, the biosensor is called a "sandwich immunoassay." In this experiment, it uses a rabbit polyclonal antibody immobilized on a polystyrene fiber waveguide. A biotin-streptavidin reaction captures Listeria cells on the fiber. The researchers confirmed the capture with a scanning electron microscope. A second antibody-cyanine murine monoclonal-then generates a specific fluorescent signal, which is activated by a 635-nanometer wavelength laser. The fluorescence is sensed by a photodetector tuned to 670 to 710 nm.

Listeria comes in six different species, but only monocytogenes infects humans, and the Purdue sensor only detects L. monocytogenes, according to Bhunia. "We think that our sensor is the only one today with the ability to distinguish just the one Listeria monocytogenes species, ignoring the other five as well as other types of foodborne contaminants, such as salmonella or Escherichia coli," he said.

Fiber-optic results were obtained in 2.5 hours of sampling in a pure culture at 37 degrees C. In typical cultures, say from a hot dog or bologna, the biosensor would require less than 24 hours to detect Listeria. "We can detect as few as 1,000 bacteria cells in a milliliter sample," said Bhunia.

In comparison, today's tests must have as many as 1 million to 10 million bacteria cells per milliliter to test positive. Unfortunately, as few as 100 Listeria cells can cause illness in someone with a weak immune system. The USDA Food Safety and Inspection Service has a zero-tolerance policy for Listeria. That means there must be absolutely no presence of Listeria-not one cell-in a food sample.

Bhunia, Geng and Morgan worked for three years developing this sensor, trying out many plastic fiber-optic materials, antibodies and fluorescent-marker molecules. The researchers predict another year of engineering effort will be required to deliver a sensor that can be used for industrial food testing. Their offering will include a remote computer-monitoring capability so that when Listeria is detected, an alarm is immediately sounded, halting food shipments long before they reach store shelves.

Today, approximately 2,500 people get listeriosis every year, according to the Centers for Disease Control and Prevention (www.cdc.gov/ncidod/dbmd/diseaseinfo/listeriosis_g.htm). Fatalities usually occur among the elderly, pregnant women, newborn infants and individuals with compromised immune systems.

Last summer, precooked chicken contaminated with Listeria was distributed to warehouses in Georgia and Arkansas and made it to store shelves in Maryland and New York before the recall was issued. The timing of the recall was delayed because the small amounts of Listeria present in the chicken had to be enriched by culturing them for seven days in petri dishes. Even DNA testing can take several days to process, but in this case the conventional enrichment test took so long that nearly 37,000 pounds of precooked chicken products had to be recalled.

According to Bhunia, suspect food that has Listeria can be sanitized by cooking it and thereby killing the bacteria. But luncheon meats and other consumables that carry Listeria often don't have to be cooked before they are eaten.

Bhunia's research group was funded by the Agricultural Research Service of the U.S. Department of Agriculture and the Center for Food Safety Engineering (www.cfse.purdue.edu) at Purdue.